FIG. 1 shows a summary of a typical MS/MS analytical method using collision-induced dissociation (CID). A first mass spectrograph (MS1) 2 selects precursor ions from among ions arriving from an ion source 1. The selected precursor ions are carried to a collision-induced dissociation chamber (CID chamber) 3, where the ions collide with a CID gas introduced from a CID gas source 4 within the CID chamber 3 and dissociate to form fragment ions. The generated fragment ions are carried to a second mass spectrograph (MS2) 5 and are detected by a detector 6. As a result, it is possible to obtain a spectrum with structural information (Patent Literature 1).
FIG. 2 is a block diagram of a flow paths used to control the flow rate of the gas introduced into the CID chamber 3. The CID chamber 3 is maintained at a medium vacuum or a high vacuum by a vacuum pump not shown in the drawing. A control valve 7 is installed immediately downstream from the CID gas source 4, and the flow path is divided into three flow paths—a main flow path 8 leading to the CID chamber 3, an atmosphere release flow path 9, and a split flow path 10—downstream from the control valve 7. A flow rate restricting resistance tube 11 and a split resistance tube 12 are disposed on the main flow path 8 and the split flow path 10, respectively, and an atmosphere release valve 13 is provided on the atmosphere release flow path 9. A pressure gauge 14 is installed upstream from the flow rate restricting resistance tube 11 of the main flow path 8. A control part 15 adjusts the degree of opening of the control valve 13 so that the gas pressure measured by the pressure gauge 14 reaches a prescribed value. The volumetric flow rate of the gas per unit time flowing into the CID chamber 3 in the standard state (20° C., atmospheric pressure) is proportional to the square of the gas pressure upstream from the flow rate restricting resistance tube 11 of the main flow path 8, so the flow rate of gas flowing into the CID chamber 3 can be controlled by adjusting the degree of opening of the control valve 13.
The CID gas is introduced into the CID chamber 3 from the CID gas source 4 through the main flow path 8, but the flow rate is extremely low (for example, approximately 0.1 cc/min in the standard state). Therefore, in the block diagram of the flow paths shown in FIG. 2, the CID gas is constantly discharged from the split flow path 10, and the volume of gas flowing into the main flow path 8 is reduced as a result. With such a configuration, the rate of change of the flow rate per unit time in the main flow path 8 is suppressed, which facilitates the control of the flow rate within a minute range.
Since the respective resistance tubes 11 and 12 are disposed on the main flow path 8 and the split flow path 10, the gas pressure on the downstream side of the resistance tubes 11 and 12 is lower than the gas pressure on the upstream side. By appropriately setting the inside diameters and lengths of the respective resistance tubes 11 and 12, it is possible to introduce a gas of a desired flow rate into the CID chamber 3. In order to control the gas flow rate into the CID chamber 3 to such a minute level after regulating the pressure of the gas discharged from the CID gas source 4 to at least atmospheric pressure (for example, approximately 300 kpa to 500 kpa), it is necessary to set the resistances of the resistance tubes 11 and 12 to extremely high levels.
In such a flow path configuration, even if the degree of opening of the control valve 7 is narrowed in order to reduce the flow rate of gas flowing into the CID chamber 3, the gas pressure upstream from the resistance tubes 11 and 12 will be reluctant to decrease. Therefore, the control part 15 releases the high-pressure gas upstream from the resistance tubes 11 and 12 via the atmosphere release flow path 9 by opening the atmosphere release valve 13 while simultaneously narrowing the degree of opening of the control valve 7. This makes it possible to instantaneously reduce the gas pressure upstream from the resistance tubes 11 and 12 and, as a result, it is possible to reduce the flow rate of gas flowing into the CID chamber 3 to a desired level in a short amount of time.